The present invention generally relates to a laser oscillator, and more particularly to a resonator used in a laser oscillator.
In a laser oscillator, it is known that the beam diameter must be changed relative to the type of outlet window when a laser beam is reflected from within a housing into the atmosphere. FIG. 6 is a sectional view illustrating the structure of a conventional CO.sub.2 gas laser oscillator having a laser beam outlet window 9 made of zinc selenide (ZnSe) material, etc.
In FIG. 6, a housing (e.g., a vacuum chamber) 1 isolates from the atmosphere a laser medium gas 2 energized by discharge energy. A mirror 3 having a reflective surface with a concave shape, and a mirror 4 having a reflective surface with a convex shape, are aligned by a line 5 (e.g., an optical axis) extending through their spherical centers. A partially apertured mirror 6 has a flat reflective surface mounted at an angle of 45 degrees relative to the line 5, and a collimating convex mirror 7 has a convex reflective surface for expanding a laser beam received from the mirror 6. A bend mirror 8 has a flat reflective surface for changing the direction of the laser beam reflected by the convex mirror 7, and a window 9 isolates the laser medium gas 2 from the atmosphere, but allows the laser beam reflected by the bend mirror 8 to exit the housing therethrough.
The operation of the laser oscillator shown in FIG. 6 is described hereinafter. In a CO.sub.2 gas laser, a gaseous mixture of CO.sub.2, He and N.sub.2 is generally sealed in the housing 1 at a negative pressure of several tens of Torr to several hundred Torr. An optical resonator comprising the concave mirror 3, the convex mirror 4 and the apertured mirror 6 is provided enclosing the laser medium gas 2 energized by discharge energy, and the laser beam is reflected and amplified between the concave mirror 3 and the convex mirror 4, passing through the partially apertured mirror 6. The laser beam from the concave mirror which "overflows" the region of the aperture impinges on a mirrored surface external to the aperture of the mirror 6 and is reflected by this reflective surface to the collimating convex mirror 7.
The collimating convex mirror 7 reflects the laser beam, and thereafter the laser beam is expanded and introduced into the window 9 via the bend mirror 8. The window 9 causes at least 99% of the CO.sub.2 laser beam to permeate (e.g., exit) therethrough and isolates the laser medium gas from the atmosphere, thereby outputting only the laser beam into the atmosphere.
FIG. 7 is a sectional view illustrating the structure of another conventional system, i.e., a CO.sub.2 gas laser oscillator wherein a laser beam outlet window is an aerodynamic or air dam window 90. In FIG. 7, characters identical to those in FIG. 6 designate identical or corresponding elements. Here, a collimating concave mirror 70 having a concave reflective surface condenses the laser beam output from the apertured mirror 6. A bend mirror 80 changes the direction of the laser beam reflected by the mirror 70, and the aerodynamic window 90 passes the laser beam reflected by the bend mirror 80 to the atmosphere. The window 90 has a small hole 91 formed in the center thereof for passing the laser beam. Atmospheric air is prevented from penetrating the housing 1 through the small hole 91 by means of a flow of gas 92 provided through the window in a direction perpendicular to the direction in which the laser beam exits the window. Hence, an air curtain is formed by air flow 92 to prevent atmospheric air from penetrating the housing 1 through the small hole 91.
The operation of the conventional device shown in FIG. 7 is described hereinbelow. The laser beam reflected by the reflective surface of the mirror 6 impinges on the collimating concave mirror 70. The laser beam reflected by the concave mirror 70 is output in a condensed (e.g., narrowed) state to the atmosphere through the small hole 91 in the aerodynamic window 90 via the bend mirror 80. The subsequent operation is similar to that of the device described in FIG. 6 and will not be described.
The known laser oscillator configured as described above requires an optical system having a collimating convex mirror and a bend mirror, when using a window made of zinc selenide (ZnSe) material, etc. for the laser beam to exit or permeate therethrough, to keep the energy density of the laser beam low with respect to the window. Further, when an aerodynamic window is employed, the laser beam must be condensed by the optical system having a collimating concave mirror and a bend mirror so that the laser beam can pass through the small hole formed in the aerodynamic window, thereby increasing the complexity of the optical system and its alignment.